There are many devices today that utilize the global positioning system (GPS). GPS is based on a constellation of twenty-four satellites orbiting around the earth that broadcast precise data signals. A single GPS receiver is capable of receiving these signals and can calculate its position (latitude and longitude), altitude, velocity, heading and precise time of day using data signals from at least four GPS satellites. Thus, these GPS receivers can locate themselves anywhere on the planet where a direct view of the GPS satellites is available.
However, a device utilizing GPS has many limitations. One significant limitation is that GPS is generally unsuitable for indoor positioning applications since a direct view of the GPS satellites is not available.